A sensitive, selective radioimmunoassay (RIA) for rat urinary kallikrein (RUK), capable of detecting 150 pg of antigen, has been developed. The RIA was used to examine immunoreactive glandular kallikrein in rat plasma. Normal male Sprague-Dawley rat plasma (n equals 6) contained 162 plus or minus 20 ng antigen/ ml (x plus or minus SEM). Bilateral nephrectomies were performed on 5 rats while 6 underwent sham operations. Analysis of the plasmas after 44 hours showed that sham plasmas contained 179 plus or minus 4.5 ng antigen/ml while nephrectomized plasmas contained 618 plus or minus 97 ng/ml (p less than .001). Gel filtration of pooled plasmas revealed 3 peaks of antigen (Ag) in both sham and nephrectomized plasmas. For both sham and nephrectomized plasmas, peak 3 showed parallel cross-reactivity to RUK while peaks 1 and 2 did not. Ag contents of the first two peaks were similar in sham and nephrectomized plasmas while the Ag content of the third peak increased about 9-fold in nephrectomized plasmas.